Aerobic metabolism continuously produces reactive oxygen species (ROS), which are often regarded as toxic mediators of cell death in pathological conditions. Studies in several cell lines, however, suggests that ROS may modulate proliferation as well as death of cells. The overall hypothesis of this study is that ROS play a central role in the enhanced proliferation and death of smooth muscle cells (SMC) in vascular diseases. We have obtained evidence that adenoviral-mediated gene transfer of the anti-oxidant enzyme catalase results in reduction in the intracellular level of H2O2 with inhibition of SMC proliferation and induction of apoptotic cell death. By using gene transfer approaches, the investigators were able to target catalase to peroxisomes, which is the normal intracellular location, instead of applying the enzyme to the outside of cells. In Aim 1, studies are now planned to test the hypothesis that endogenous H2O2 regulates proliferation and viability of SMC. The investigators propose to use pharmacologic and gene transfer approaches to determine the importance of catalase enzymatic activity in modulating cell proliferation and viability, and to examine H202-metabolizing enzyme, glutathione peroxidase (GPx), to specific subcellular locations to test effects on cell proliferation and viability. In Aim 2, the investigators propose to test the hypothesis that O2 , and the balance between O2 and H2O2, regulates proliferation and viability of SMC. The investigators propose to pharmacologic and gene transfer approaches to determine the importance of catalase enzymatic activity in modulating cell proliferation and viability, and to examine H2O2-metabolizing enzyme, glutathione peroxidase (GPX), to specific subcellular locations to test effects on cell proliferation and viability. In Aim 2, the investigators propose to test the hypothesis that O2, and the balance between O2 and H2O2 regulates proliferation and viability of SMC. Gene transfer of Cu/Zn superoxide dismutase (SOD), MnSOD, or extracellular SOD, along with catalase or GPx, will clarify the relative contributions of O2 and H2O2. In Aim 3, the investigators present data suggesting that ROS-induced cytotoxicity is enhanced in neointimal as compared with medial SMC, a finding which may have important implications in regard to vascular remodeling and plaque rupture. The hypothesis is that the enhanced ROS-induced toxicity is due to impaired oxidant defense mechanisms and/or augmented productions of ROS specific to the phenotype of neointimal SMC cultures derived from balloon-injured rats. The proposed studies should provide novel insight into oxidative mechanisms that regulate growth and visibility of SMC.